I think you've misunderstood his post (or I've misunderstood yours (my friend))). He was saying that the linked website was illogical in using a method to distinguish humans from machines, that most humans would fail at. Grandparent wasn't attempting to construct his own logical argument.

But the logic of the CAPTCHA is "if you get this right, then you are human". Makmanalp was suggesting "if you get this wrong, you are not human", which does not follow logically from the CAPTCHA rule.

It is an example of "Denying the antecedent", or the inverse error: http://en.wikipedia.org/wiki/Denying_the_antecedent

Who is to say that "if you get this right, then you are human" is the valid premise? I'd argue that the correct one is "If you're human, you get CAPTCHAs right". :P

Just to be pedantic: CAPTCHAs are a test for being human so human-ness has to be the consequence of some condition. Your premise is therefore an incorrect framing of the CAPTCHA test.

(IsaacL, I think we are all having our tongue in our cheek here).